1. Field of the Invention
The present invention relates to a method for producing a chip-type coil device and, in particular, to a method for producing a wire-wound chip-type coil device in which a wire is wound around a core of the coil device. The invention also relates to a chip-type coil device produced by this method.
2. Description of the Related Art
FIG. 8 is a perspective view of a conventional chip-type coil device 1. This chip-type coil device 1 can be used, for example, to provide a high-frequency coil.
The chip-type coil device 1 shown in FIG. 8 includes a core 6. The core 6 has a winding section 2 and leg portions 4 and 5 protruding from end portions of the winding section 2. The gap between the leg portions 4 and 5 defines a recess 3. Terminal electrodes 7 and 8 are formed at the ends of the leg portions 4 and 5, respectively. A wire 9 is wound around the winding section 2, and the ends of the wire 9 are electrically connected to the terminal electrodes 7 and 8 by soldering or the like.
The above-described chip-type coil device 1 is produced by first forming the core 6. The core 6 is produced by loading a ferrite powder material, an alumina powder material, a dielectric powder material or the like into a press forming machine 10, as shown in FIG. 9, and thereafter firing the material. The press forming machine 10 is equipped with dies 13 and 14 which operate in the directions of arrows 11 and 12 to exert a pressing force on the material disposed therebetween.
After forming the core 6 in this manner, the terminal electrodes 7 and 8 are formed on the end portions of the leg portions 4 and 5 of the core 6. The terminal electrodes are formed by plating, printing or the like, in combination with a masking operation. Next, the wire 9 is wound around the winding section 2 of the core 6, and the ends of this wire 9 are connected to the terminal electrodes 7 and 8, thereby completing the chip-type coil device 1.
However, the above-described method of producing the chip-type coil 1 device has a number of disadvantages. First, to reduce the size of the chip-type coil device 1, it is necessary to reduce the size of the core 6. As the size of the core 6 is reduced, the press forming machine 10, including dies 13 and 14, must exercise a higher level of precision in its performance. This makes it more difficult to control the powder material charging amount and the press pressure in the press forming machine 10. This, in turn, places substantial limitations on the extent to which this type of machine can be improved. Further, this technique is not suitable for mass production, which increases the cost of manufacturing the chip-type coil devices.
Moreover, in terms of performance, press forming the core 6 as shown in FIG. 9 tends to cause an imbalance in density in the core 6. Note FIG. 9, for instance, which shows the density of the core 6 after press forming, as represented by the density of dots. For example, the density in portion 15 is relatively high and, in portion 16, it is relatively low. As a result, after firing, the dimensional accuracy of the core 6 is rather poor. Further, this imbalance in density produces a variation in the strength of the core 6.
Further, as the size of the core 6 is reduced, the terminal electrodes 7 and 8 become smaller, making it more difficult to form these terminal electrodes 7 and 8 with high dimensional accuracy. In the case of a high-frequency coil device, the variation in the value of capacitance (stray capacitance) due to the variation in the dimensions of the terminal electrodes 7 and 8 considerably affects the high-frequency characteristics of the device, so that a high level of dimensional accuracy is required for the terminal electrodes 7 and 8.